Most of the information I’ve seen on PHP opcode caches focus on the performance aspect — having the opcode precompiled and ready — but the other benefit I’ve found is that process sizes are reduced dramatically. This may not be apparent at first, if you don’t consider the shared memory used by each process. For example, before activating APC, my httpd process size was about 100 MB in memory (RSS), with about 20 MB of that being shared (SHR) between all httpd processes. This means the actual RAM used by the httpd process was about 80 MB, not 100 MB. After installing APC, the process size in memory (RSS) has increased to about 120-140 MB, but the shared part of that memory has increased even more -- it's now about 50-60% of the RSS size. This means the actual / real process size has gone from 80 MB down to about 60 MB or less!